1. Field of the Invention
The present invention relates to a method for preparing highly pure 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as "bisphenol A").
More specifically, the present invention pertains to a method for preparing bisphenol A which comprises reacting phenol and acetone in the presence of an acidic ion-exchange resin as a catalyst and which is characterized in that the reaction is performed while a part of the water generated during the reaction is removed from the reaction mixture containing phenol and acetone through a separating membrane.
2. Description of the Prior Art
Bisphenol A has been used as a starting material for producing polycarbonate resins and epoxy resins and recently demands thereof as a starting material for preparing engineering plastics or the like has gradually been increased.
It has been known that highly pure bisphenol A must be used in such applications. In particular, when it is used as a starting material for polycarbonate resins, bisphenol A must be colorless and highly pure. On the other hand, a method for economically producing bisphenol A has been desired as the demands thereof increased.
Bisphenol A has been prepared by reacting acetone with an excess of phenol in the presence of an acidic catalyst and optionally a co-catalyst such as a sulfur compound. The reaction may be batchwise or continuously performed.
As such catalysts in this condensation reaction, there have generally been used those catalysts soluble in the reaction system such as hydrochloric acid or sulfuric acid. However, the use of apparatus of an expensive material is required in order to use these corrosive acidic catalysts and these catalysts (acids) must be removed from the reaction mixture through a large scale purification process. This makes these methods economically unfavorable. Moreover, the quality of the resulting product is greatly impaired even if a trace amount of these acids remains in the product.
A solid acidic catalyst may be employed as a catalyst for preparing bisphenol A. When such a solid acidic catalyst is used, the foregoing problem of corrosion can be solved to some extent. Moreover, this results in the simplification of the production process since a reactor equipped with a fixed bed or a fluidized bed as the catalyst layer can be adopted and correspondingly the complicated process for removing the catalyst can be omitted unlike the cases wherein a soluble catalyst is employed.
It has been known that acidic ion-exchange resin can be used as such a solid acidic catalyst (see, for instance, Japanese Patent Examined Publication (hereunder referred to as "J.P. KOKOKU") No. Sho 36-23334).
The reaction can be accelerated by the addition of a co-catalyst such as a sulfur atom-containing compound, e.g., those carrying a mercapto group even when an ion-exchange resin is employed as the catalyst (see J.P. KOKOKU No. Sho 46-19953). In this case, the mercapto group is fixed to the ion-exchange resin in the form of a compound having a nitrogen atom on the side opposite to the mercapto group. In these methods, the reaction is performed using phenol in an amount of 4 to 10 times the molar amount of acetone at a temperature ranging from 40.degree. to 100.degree. C. and the ion-exchange resin is dried prior to use since the lower water content of the reaction system is preferred.
The method for preparing bisphenol A from acetone and phenol in the presence of an ion-exchange resin catalyst suffers from a problem that the catalytic activity of the ion-exchange resin is greatly influenced by the water formed almost stoichiometrically during the reaction with the formation of the desired product, bisphenol A, irrespective of whether the ion-exchange resin is modified with an amine carrying a mercapto group or not.
The amount of the water generated during the reaction reaches not less than 1.4% at the time when the acetone conversion reaches 50% even if the reaction is performed at a molar ratio, phenol/acetone, of 6:1.
For this reason, the reduction in the catalytic activity of the ion-exchange resin due to the presence of water formed starts as soon as the reaction is initiated even if the ion-exchange resin catalyst has been dried prior to use.
Moreover, if a mineral acid catalyst soluble in phenol such as hydrochloric acid or sulfuric acid is employed, the acetone conversion generally reaches 95% or more within 6 to 8 hours. On the other hand, if an ion-exchange resin catalyst is used, the reaction rate is gradually lowered since the amount of water present in the reaction system increases as the reaction proceeds. As a result, it takes 10 hours or more to achieve a conversion of 95% or more in the batch reaction system even if a large amount of a catalyst is employed and it is substantially impossible to achieve a conversion of the order of 95% or more in the continuous reaction system.
Therefore, if a fixed bed of an ion-exchange resin is used, the problems associated with the case wherein a soluble inorganic acid catalyst is used such as the removal of the catalyst from the reaction system and the contamination of the remaining acid and/or inorganic substances into the result product can be solved, but the use of the fixed bed of the resin suffers from the problems such as the lowering of the reaction rate due to the water generated through the reaction, the corresponding lowering of the acetone conversion and the necessity of complicated processes associated with post-treatment.
If it is intended to increase the acetone conversion up to 90% or higher according to a conventional technique in which a fixed bed system is adopted, a large amount of an ion-exchange resin must be employed. This is unpracticable from the industrial viewpoint.
Accordingly, it is stated in J.P. KOKOKU No. Sho 36-23334 that the conversion of acetone is preferably about 50%. In this patent, the unreacted acetone is separated from the reaction system together with a part of the water generated and a part of phenol and then isolated through complicated processes which require a large amount of separation energy for recovering and reusing it.
However, the concentration of bisphenol A in the resulting flow of the reaction product is low if such a method is adopted and the method in turn requires excess energy for the separation and purification of the product and is not considered to be an economically effective process.
Alternatively, a method is also known which comprises the steps of removing acetone, water and a part of phenol from a part of the flow of the reaction product and recycling a part thereof to the reaction system to thus increase the concentration of bisphenol A in the flow of the reaction product (see Japanese Patent Unexamined Publication (hereunder referred to as "J.P. KOKAI") No. Sho 54-19951), but any means for recovering the unreacted acetone is not disclosed in the patent, the throughput capacity of the reactor is substantially impaired in this method and, therefore, this is not an economically effective method.
J.P. KOKOKU No. Sho 63-52021 discloses a method for preparing bisphenol A which is characterized in that the reaction is peformed while a mixed solution containing phenol and acetone is simultaneously or alternatively brought into contact with an acidic ion-exchange resin and a dehydrating agent. According to this method, the acetone conversion can be increased to almost 100% without recycling the flow of the reaction product. However, the dehydrating agent must, in any case, be regenerated before the dehydrating agent is saturated with the water formed during the reaction and, therefore, this method requires the use of a large-scale apparatus and complicated operations in order to carry it out on an industrial scale.